Anyone know how to care for Imported Pinus Parv. in Zone 7?

I bought a White Pine, grafted in Black Pine root stock, from a nursery in Shanghai, China this Spring. The tree is 40 years old and is growing very well. It is budded up for Spring. I Winter my trees on the ground, in mulch which covers the pots. They are up against the sides of the garage, three deep at most and have some wind protection. I read about Winters in Shanghai and am wondering if my "way" to Winter this tree is sufficient. Or should I build a cold frame or something? I am in Zone 7. I believe Shanghai is Zone 7-8, depending upon the micro climate... Does anyone have suggestions or thoughts about this?
Thanks you for your help. Todd

I suspect the tree will be resilient enough to take the slight differences between zone 7-8 temps. MY concern would be having the trees on the ground. Every time I've tried that, the squirrels and other little chewing critters have tried to restyle my trees.

I'm in the border between zone 7-8, according to the new USDA map.

Thanks Jim. So far, the squirrel "nuisance" has been minimal with my trees. This could change though. We are having a bumper crop of hickory and acorn nuts this year... which means pulling out seedling oaks next year... fun, fun fun... NOT! These oaks have taproots to Shanghai!
Todd